Liquid crystal display devices, which are common display devices, are required to have high display performance and durability and expected to display images in excellent contrast and tone balance at a wide viewing angle. Such requirements have been met through the use of liquid crystal panels conforming to various display modes for liquid crystal display devices, for example, the VA (vertical alignment) mode, the OCB (optically compensated bend) mode, and the IPS (in-plane switching) mode. Such liquid crystal panels have wider viewing angles and higher display performance compared to those of liquid crystal panels conforming to the conventional TN (twisted nematic) mode.
Along with the increasing demand for energy efficiency, there also has been an increasing demand for display devices with wide viewing angles and high display performance. In such view, display devices including organic electroluminescent (hereafter, it is abbreviated as an organic EL) backlights have been drawing attention as next-generation display devices conforming to a new display mode.
An organic EL display device includes pixels provided with light sources that can be independently turned on or off. Thus, power consumption is low compared to that of liquid crystal display devices, which include backlights that are always turned on during image display. The control of transmission and non-transmission of light through each pixel in an image displayed on liquid crystal display devices involves a liquid crystal cell and polarizing plate disposed on both sides of the liquid crystal cell; whereas organic EL display devices do not require such a configuration because images can be formed through turning on and off the light sources, and thus can have significantly sharp front contrast and a wide viewing angle. In particular, the use of organic EL elements of the colors blue (B), green (G), and red (R) eliminates the need for color filters, which are essential for liquid crystal display devices; thus, organic EL display devices are expected to achieve higher contrast.
A typical organic EL display device includes a reflector having a mirror surface on the surface opposite to the light-extracting surface in the form of a highly reflective metal material serving as an electrode layer constituting the cathode or a separate metal plate serving as a reflector, to efficiently transmit light from a light-emitting layer to the viewed surface.
Unfortunately, unlike liquid crystal display devices, organic EL display devices do not include crossed Nicol polarizers; thus, external light is reflected by the light-extracting reflectors and forms a reflection, causing a significant decrease in contrast in a high brightness environment.
To solve such a problem, for example, a countermeasure is disclosed involving a circularly polarizer element for prevention of reflection of external light by a mirror surface (for example, refer to Patent Document 1). The circularly polarizer element described in Patent Document 1 includes an absorptive linear polarizing plate and a λ/4 retarder film, which are laminated such that their optical axes intersect at 45° or 135°.
A conventional retarder can adjust the retardation of a monochrome light beam to λ/4 or λ/2 of the wavelength of the light beam, but converts white light, which consists of combined waves of various visible light beams, into a spectrum of colored light polarized in accordance with the different wavelengths. This is because the material of the retarder exhibits wavelength dispersion corresponding to the phase difference.
To solve such a problem, various wideband retarders have been studied to achieve uniform retardation of light beams over a wide wavelength band. For example, a retarder includes a λ/4 wave plate that retards birefringent light by ¼ of the wavelength and a λ/2 wave plate that retards birefringent light by ½ of the wavelength, which are bonded together such that their optical axes intersect (for example, refer to Patent Document 2).
The production of the retarders described above requires a complicated step of adjusting the optical direction (optical axis or slow axis) of two polymeric films and a step of bonding multiple films with an adhesive layer, which hinders the advantage of organic EL display devices of being thin; thus, there is a need for the development of a wideband λ/4 retarder having a non-laminated single layer configuration.
Similar to the liquid crystal display device, an absorptive linear polarizing plate in a circularly polarizing plate described above is typically composed of polyvinyl alcohol (hereafter, it is abbreviated as PVA) containing dichroic pigments and stretched to a length much greater than the original length; such a polarizer film is readily affected by the external environment, and thus requires a protective film. A widely used protective film for polarizer elements is composed of cellulose, for example, cellulose ester, which has excellent adhesiveness to PVA in the form of a polarizer element and high total light transmittance. Thus, the polarizing plate includes a polarizer element and polarizer protective films disposed on both sides of the polarizer element, and must also include a λ/4 retarder film so as to function as a circularly polarizing plate.
The λ/4 retarder film disposed on the polarizing plate protective film causes the retardation to deviate from λ/4, which is a desired optical property, due to the slight retardation ability of the polarizer protective film, and the increased number of components causes an increase in the thickness; thus, there is a demand for the development of an optical film that can function as both a polarizer protective film and a wideband λ/4 retarder.
A technique for producing a monolayer wideband λ/4 retarder film is disclosed. The λ/4 retarder film is produced through uniaxial stretching of a copolymer film composed of polymerized monomers having positive refractive-index anisotropy and monomers having negative birefringence (for example, refer to Patent Document 3). The uniaxially stretched polymeric film has inverse wavelength dispersion, which enables the production of a wideband λ/4 retarder from a single retarder film. Unfortunately, the polarizing plate protective film has poor adhesiveness to a polarizer element and insufficient total light transmittance.
The application of an optical film functioning both as an optical compensator and a polarizing plate protective film to a liquid crystal display device has been investigated. As such a film, an optical film consisting of a cellulose ester film having a predetermined retardation has been studied. For example, an optical film in the form of a retarder film conforming to the VA mode is disclosed. The retarder film is composed of cellulose ester having an in-plane retardation Ro of approximately 50 nm and a retardation Rt across the thickness of approximately 130 nm (for example, refer to Patent Document 4).
Cellulose ester is characterized in that a decrease in the degree of substitution relatively increases the phase difference but decreases the inverse wavelength dispersion, whereas an increase in the degree of substitution increases the inverse wavelength dispersion but decreases the retardation. Thus, a monolayer wideband λ/4 retarder can only be produced with a large thickness.
Other techniques have been investigated for an enhancement in the retardation and the wavelength dispersion of a film through the addition of additives, such as retardation enhancers and wavelength dispersion adjusters, to cellulose esters. Unfortunately, a large amount of additives impairs the quality of the film, causing a decrease in durability and transparency; thus, a solution to this drawback is required.
To solve the issues described above, a technique has been studied for the enhancement in the wavelength dispersion of a cellulose ester film through introduction of specific aromatic ester groups to cellulose ester (for example, refer to Patent Document 5). The technique proposed in Patent Document 5 can freely control the wavelength dispersion of a cellulose ester film without causing a decrease in the retardation ability.
The present inventors have conducted an extensive study on the technique proposed in Patent Document 5 and have identified a problem of unevenness in tone and reflection of displayed images that occurs depending on the use environment when a wideband λ/4 retarder film is used as a circularly polarizing plate for an organic EL display device, which is produced through control of the substituents of cellulose ester described in Patent Document 5 so as to adjust retardation and wavelength dispersibility corresponding to phase difference.
Accompanied by the introduction of a thinner thickness of an organic EL display device, the employing environment thereof has been enlarged and the applications thereof have been diversified. It was found that an organic EL display device was particularly prone to the problem described above when humidity fluctuated in the use environment; thus, the need for immediate measures for improvement was apparent.
Further, it was found that a cellulose ester film had a high hygroscopic property and had a large variation of retardation value according to the change of environmental humidity. With respect to this problem, it has been examined a method of decreasing the change of the optical property which depends on the environmental humidity by incorporating a specific additive in a cellulose ester film.
Patent Document 6 discloses a method using a cellulose ester film which incorporates a compound having a value obtained by dividing a molecular weight with a sum of a hydrogen bond donor number and a hydrogen bond acceptor number in the specific range.
Patent Document 7 discloses an example of adding a benzoic acid derivative, and Patent Document 8 discloses an example of adding a nucleic acid derivative to a cellulose ester film as a humidity resistance improving agent.
Patent Document 9 discloses a method using a cellulose ester film which incorporates a high moisture absorptive compound having a water content difference of 2% or more.
The present inventors examined the methods each described in Patent Documents 6 to 9 under the severer environmental conditions than before. It was found that although the methods described in Patent Documents 6 to 9 showed certain improvement under mild environmental conditions, the durability was still insufficient for the purpose of using for a high-grade display device recently developed, and that further improvement was still required.
That is, the film may be under the severe environment such as directly exposed to water by dew condensation during transportation of the film. Even in this environment, it has become required a property of showing no property variation. It was found that the known methods described in Patent Documents 6 to 9 showed only small improvement effect under the severe conditions such as directly exposed to water as described above.